why does grad school feel infinitely harder on the psyche than undergrad?

In graduate school, I feel like I have less to worry about than in undergrad and yet it feels far more strenuous on my mental health. Undergrad was also engineering but idk if it was the freedom of not working under someone or what but it was farrr less stressful for me than grad school has been. Its been research and class work. I had to ta at one semester but that part is behind me.